My friend has a good 1968 390 FE HP motor . Can it be stroked to a 428 and what build combo with the stock heads & hyd cam will make the most horse power ? It will go into a 1968 Mustang C6 3.90 gear wild street car has manual brakes .

I am not an FE Ford Expert . But I have worked on plenty 361-391 FT Engines . And I use to Help a Friend swing in a bunch of 390's and 428's in his 1970 Shelby GT-500 . He Blew up a nice Low mile 428 SCJ Road Racing . That 390 with a 428 Crank and Rods can go to a 410 with the correct pistons a Bit more with +.030 410 Pistons maybe 416 ci . As for going out on the Bore from 4.05" to 4.13" 428 Bore Size . I think if the Bore thickness is checked it should be fine to do then you will have your 428 . I would see what Pistons are cheapest and try to take the build in that direction . As for most H.P ? . Could start a War with that one . I would put it under 500 H.P with un-Ported Ford Iron Head's . Our Ford Tech knows these Engines way better then I . I am sure he will add more info to this thread .

Tango is right on the 410+ cu. This would be the most cost effective route. Good 428 cranks normally go for $300 to $450. At $450 it should be polished or undercut and not more than .010 if so. They have come down a bit in price over the last few years. OEM 428 cranks will be stamped on one of the counterweight edges with a 1U or 1UB. Those are both the std 428 and CJ cranks. There's also the SCJ 1UA but those are somewhat rare and normally a whole lot more money when you find them. You can use the stock 390 rods and you'd need a 410 piston. There are some good piston options like stock cast 410 by SpeedPro. Hypers by Keith Black and Probe, Diamond and others sell nice forged pistons. You can also use stock low comp 390 truck pistons. These are the same as the orignal 66-67 410 piston, same molds just new part #. The pin height was moved up on the 410 so the short FE rods gave good comp height with both low comp 390 engines and 410-428 stroke cranks. This piston was the choice for the low compression 390 truck engines as no new part needed to be drawn up. If you can find a decent set of used forged 390 pistons you can shave the top on some for zero deck height. TRW has some that are pretty popular. I can get you part numbers for any of these if needed. Also the oem 410-428 cranks are external balance and need a 410-428 flywheel with the balance weight on it. Lots of vendors sell street to SFI rated flywheels $80-300. You can also choose to internal balance the assy. 390's are factory internal balanced and use a flywheel without a counterweight. So you'll need to address which balance you'd prefer here as well.

As far as boring the block to 4.13 std 428 a sonic map properly done will tell you the thickness. There is a simple field test called "the drill bit test". This is where you simply gap the cyl wall cores inside the freeze plugs. This test was invented/created by Dave Shoe. Dave chose 64" increments so you can convert those into the bit sizes you have available. You simply stick the smooth shank end of the drill bit betrween the cyl cores to gauge the gap. The smaller the bit the thicker the walls. 16/64" - 17/64" bit shank fits 390 wall cores and 13/64" - 14/64" (1/8") fits 428 wall cores. Bit should be a snug fit. And again covert the 64's to suit your bit sizes. But this will give you a very good idea what your starting with.

There are also aftermarket stroker kits by Scat and others. You can get 3.98, 4.125 and 4.250 stroke cranks. The 4.250 with 4.05 390 bore will net you 445+ cubes depending on exact overbore. These cranks use a BBC size rod journal and longer 6.7" BBC rods. Many vendors sell these kits complete. Normally prices are $1600-1900+. Cheaper kits you find are often not balanced which adds $200-300. Good kits also include rod and main bearings and piston rings. Besides the crank, rods and pistons. So be sure to ask and compare what's included. I can recomend a few good vendors. Scat FE cranks are normally internal balance and take a slug or 2 of mallory.

The heads are more than likely stock C8AE-H heads. These heads can have a 8-14 or 16 bolt exhaust manifold pattern. A non issue if using headers but you'll want the 14-16 hole heads if you choose stock manifolds in the Mustang. I would use headers if your doing a stroker. Ported C8's with some larger CJ size valves will run you $800-1200 depending on the vendor/shop rates. Stock Edelbrock aluminum heads are like $1500. You can also get a pair of oem iron 427 low riser or 428 CJ for $600-800 normally in good shape. These already have larger valves 2.19 and 1.65. C4AE-G and C6AE-R heads are a cheaper choice $200-400 core price nice and clean. These use stock 2.09 and 1.50 valves like the C8-H's. So again time to shop and compare. As you see this can get expensive quick.

You can also do a simple stock rebuild on the 390 with a nice cam, intake and port match and headers and be 375+ hp. This would save a ton of cash if your on a tight budget. If that 390 block is already at .030 and needs a overbore you'd want to think it over. If it cleans up at .040 it may be ok but any more overbore and coreshift might leave the walls thin at .060. At that point I'd look for another core engine. There are many options and combo's it pretty much endless. So gather a good plan and budget and see which way best suits you. And we can go from there. The main thing with a FE engine is you want to be sure of what your buying. We can check all numbers and stampings so you know exactly what parts your getting. For example that 390 could be a 360 with a shorter stroke crank and longer rods. You just never know until you check. Blocks are hard to ID by casting number so by measure is the best way there. Everything else can be ID'd by part number.

The more HP and TQ you want will choose the path. You can easily drop $6000-8000 into a 390/445 stroker build. For that coin and less you can build a stronger crossbolted 427 block based engine. The 427 is pretty much the king daddy of the FE blocks. Again many, many options.

You'll have a trans crossmember change with the big block and the transmission/flywheel/clutch is another area to look at too. Adds up fast. LOL